Mechanical strains within a semiconductor device substrate can modulate device performance by, for example, increasing the mobility of the carriers in the semiconductor device. That is, strains within a semiconductor device are known to enhance semiconductor device characteristics. Thus, to improve the characteristics of a semiconductor device, tensile and/or compressive strains are created in the channel of the n-type devices (e.g., NFETs) and/or p-type devices (e.g., PFETs), respectively. However, the same strain component, for example tensile strain or compressive strain, improves the device characteristics of one type of device (i.e., n-type device or p-type device) while discriminatively affecting the characteristics of the other type device.
Accordingly, in order to maximize the performance of both NFETs and PFETs within integrated circuit (IC) devices, the strain components should be engineered and applied differently for NFETs and PFETs. That is, because the type of strain which is beneficial for the performance of an NFET is generally disadvantageous for the performance of the PFET. More particularly, when a device is in tension (in the direction of current flow in a planar device), the performance characteristics of the NFET are enhanced while the performance characteristics of the PFET are diminished.
To selectively create tensile strain in an NFET and compressive strain in a PFET, distinctive processes and different combinations of materials are used. For example, liners on gate sidewalls have been proposed to selectively induce the appropriate strain in the channels of the FET devices. By providing liners the appropriate strain is applied closer to the device. While this method does provide tensile strains to the NFET device and compressive strains along the longitudinal direction of the PFET device, they may require additional materials and/or more complex processing, and thus, result in higher cost. Further, the level of strain that can be applied in these situations is typically moderate (i.e., on the order of 100s of MPa). Thus, it is desired to provide more cost-effective and simplified methods for creating larger tensile and compressive strains in the channels of the NFETs and PFETs, respectively.
To increase the strain levels in a device, an epitaxially grown SiGe layer has been used. When epitaxially grown on silicon, an unrelaxed SiGe layer will have a lattice constant that conforms to that of the silicon substrate. Upon relaxation (through a high temperature process for example), though, the SiGe lattice constant approaches that of its intrinsic lattice constant which is larger than that of silicon. Accordingly, when a silicon layer is epitaxially grown thereon, the silicon layer conforms to the larger lattice constant of the relaxed SiGe layer which results in a physical biaxial strain (e.g., expansion) to the silicon layer. This physical strain applied to the silicon layer is beneficial to the devices (e.g., CMOS devices) because the expanded silicon layer increases N type device performance and a higher Ge concentration in the SiGe layer improves P type device performances.
Relaxation in SiGe on silicon substrates occurs through the formation of misfit dislocations. The misfit dislocations facilitate the lattice constant in the SiGe layer to seek its intrinsic value by providing extra half-planes of silicon in the substrate. The mismatch strain across the SiGe/silicon interface is then accommodated and the SiGe lattice constant is allowed to get larger. However, misfit dislocations formed between the SiGe layer and the silicon epitaxial layer are random and highly non-uniform and cannot be easily controlled due to heterogeneous nucleation that cannot be easily controlled. Also, misfit dislocation densities are significantly different from one place to another. Thus, the physical strain derived from the non-uniform misfit dislocations are apt to be also highly non-uniform in the silicon epitaxial layer, and this non-uniform strain causes non-uniform benefits for performance with larger variability.
Accordingly, there exists a need in the art to overcome the deficiencies and limitations described hereinabove.